LPRs you ask? License plate readers, like the ones found on the two sole roads leading in and out of Tiburon, Calif., that can feed police a list of plate scans at up to 60 per second. Police use the scanners to spot stolen cars or people who might be wanted, but privacy advocates are raising concerns about everybody else. Tiburon has been using LPRs since 2009, and now similar readers can be found all over the country.

Ars Technica reports:

In late July 2012, the American Civil Liberties Union and its affiliates sent requests to local police departments and state agencies across 38 states to request information on how LPRs are used.

Law enforcement officials in Tiburon say crime has dropped one-third since the cameras have been up and running. The nation’s capital sports the most densely packed area of LPRs, sporting more than one every square mile. A major privacy concern that keeps popping up is data retention. Again from Ars:

"I want to give law enforcement all the tools to catch the bad guys, but I don't want to yield to [a] Big Brother state," Utah State Senator Todd Weiler, a Republican, told Ars. "There has to be a happy medium, but part of the happy medium is showing how long you need to store the data."

So far, lawsuits challenging LPRs are being upheld.